Updated: Wednesday, October 12th, 2016 at 12:30pm

DETROIT — The federal government has accused a small air bag parts maker of stonewalling an investigation into a fatal car crash, and threatened a large fine over the lack of cooperation.

The National Highway Traffic Safety Administration said ARC Automotive Inc. of Knoxville, Tennessee, has missed deadlines and failed to report crash information and test results as required by law. The allegations are contained in documents made available on the agency’s website late Tuesday.

The failures “raise serious questions regarding the quality and integrity of ARC’s air bag inflators,” the agency wrote in an Oct. 4 letter to the company.

Email and telephone messages left for ARC were not immediately returned.

NHTSA began asking for detailed information in August after an ARC air bag inflator ruptured and killed a woman in Newfoundland, Canada. As many as 8 million ARC inflators are under scrutiny. The investigation is separate from one that resulted in the recall of 69 million inflators in the U.S. made by Takata Corp. of Japan.

Authorities say the Canadian woman was killed July 8 when the ARC inflator ruptured and sent metal shrapnel into the passenger compartment of 2009 Hyundai Elantra she was driving. Without the shrapnel injuries, she likely would have survived the low-speed crash, Canadian officials said. Now, U.S. and Canadian investigators are trying to figure out what caused the inflator to blow apart.

With multiple deaths already confirmed from inflators made by Takata, the death involving a different inflator raised questions about the safety of air bags, which rely on explosions to fill bags that protect people in crashes. It also brought new urgency to a probe opened last year by NHTSA after an Ohio woman was injured by an ARC inflator.

NHTSA has said it wants to determine the entire population of ARC inflators in the U.S., which it estimates at 8 million mainly in older vehicles made by General Motors, Fiat Chrysler, Hyundai and Kia. About 2.6 million of the parts were sold to General Motors.

In the Oct. 4 letter, NHTSA accused ARC of missing multiple deadlines to provide data and test results showing inflator failures, failing to report a recall done by Toyota due to an ARC inflator defect, and failure to comply with directives from the agency.

ARC has questioned whether it needs to give NHTSA the information has failed to provide documents in a readable format and has “appeared nonchalant” in developing a plan to test the inflators, the letter said.

“Instead of noting the serious nature of these incidents earlier this year and committing to work with NHTSA to determine the appropriate range of issues at hand, ARC’s counsel stated that they had no obligation to provide such information and chastised agency staff for indicating otherwise,” said the letter from Michael Brown, acting director of NHTSA’s Office of Defects Investigation.

The agency threatened to hold a public hearing and fine the company up to $21,000 per day to a maximum of $105 million.

It also said the company failed to file a legally required report on the fatality in Canada, and that NHTSA found out about the death from reports by Hyundai and Canadian safety regulators.

According to NHTSA, the Elantra in the Newfoundland crash had an ARC inflator that was made in China, but it’s unknown whether any of the same inflators were used in other U.S. vehicles. ARC has confirmed that the Canadian Elantra inflator “was substantially the same design” as the one used in at least one other U.S. model, the 2004 Optima, the agency said.

Hyundai said the 2009 Elantra sold in the U.S. has a two-stage air bag inflator, not a single-stage one like the Canadian version. The automaker said it’s cooperating with investigators.